The subject of truth in this case is whether the stuff mentioned in the scripture is real or not. The basis is based primarily on tangible evidence, followed by written evidence, followed by logical reasoning.
Indian civilization is at least in the range of 200,000 years to 400,000 years old according to Attirampakkam and Narmada Human. In fact, Arun Sonakia, who discovered the Narmada Human dates it to be ~600,000 years old. And some date the tools in Attirampakkam to be ~1.5 million years old.